Transducer drive rod



July 8, 1969 A. MORRISON TRANSDUCER DRIVE ROD Sheet Filed April 28, 1967Tic}. 1. 22

N 0 mm W V 0 WM 4 J w July 8, 1969 A. MORRISON 1 TRANSDUCER DRIVE RODFiled April 28, 1967 Sheet 5 of s I N VE NTOR. 100/3 4, Name/w y 1959 L.A. MORRISON 3,454,912

TRANSDUCER DRIVE ROD Filed April 28, 1967 Sheet 3 of s h M -H. H W V v\I l FFfQUE/VCV //V CYCLES PEI? J'ECO/VD i E INVENTOR.

a v 100/: A Mam/J0 BY 4 m K 4 v ,4770XA/ 3,454,912 TRANSDUCER DRIVE RODLouis A. Morrison, Madison, N.J., assignor to Roanwell Corporation, NewYork, N.Y., a corporation of New York Filed Apr. 28, 1967, Ser. No.634,571 Int. Cl. H01f 7/08 US. Cl. 335231 2 Claims ABSTRACT OF THEDISCLOSURE BACKGROUND, OBJECTS AND SUMMARY OF THE INVENTION Thisinvention relates to an electro-acoustic transducer and moreparticularly to a type of transducer known as the balanced armaturetype. Such a transducer is capable of serving as the active element ineither a transmitter or a receiver.

The present invention is particularly concerned with the dynamicelements, that is, the armature-drive roddiaphragm combination in abalanced armature type of transducer and the invention constitutes animprovement in the drive rod design so as to obtain operating efficiencyfor the diaphragm in such a transducer.

Within the compass of the designation, balanced armature there happen tobe a number of slightly different transducers. One of these is termed areed armature and basically consists of an armature which is clamped atone end and is free to vibrate at the other end in a magnetic field.Typically this armature is produced in an E-shape although a U-shape mayalso be used. Considering the E-shape armature, the central or middlearm of the E is the vibratable arm. This reed armature type oftransducer may be appreciated by reference to the patent to Carlson3,111,563. In the transducer described in that patent the clamped end ofthe armature forms the spring which supplies the restoring force to thecentral vibratable arm when it is deflected from its rest position.

The other basic type of balanced armature transducer is one which isdesignated rocking armature, in which the armature is supported by aspring located, for example, at the middle of the armature. It is thistype of balanced armature transducer which forms the context or ambiencein which the concept of the present invention is most judiciouslyexploited. In order to furnish background material for a completeappreciation of the present invention, reference may be made to thepatent to Blount 2,267,808 in which there is described a rockingarmature type of electromagnetic transducer.

In the rocking armature transducer, both ends of the armature are freeto vibrate in the magnetic fields and the spring which supplies therestoring force is not necessarily of the same material as the armature.Thus, comparing the two types, the rocking armature type (or 4-air gapstructure) as described in the Blount patent, has the stiffness of itsdynamic elements supplied by a spring material which has good elasticproperties, while in the reed armature type as described in the Carlsonpatent, the spring stiffness is supplied by the armature itself, and

United States Patent hence is usually supplied by a material having poorelastic properties. In addition, the output of the rocking armature typeis at least theoretically, 6 db greater than for the reed armature type.

It will be apparent from the preceding discussion that the rockingarmature type of electromagnetic transducer is almost ideally suited forapplication to, for example, hearing aid receivers or other tiny headphone devices where extremely small sized elements having largeacoustical outputs are required. Of course, this is not to say thatbalanced armature electromagnetic transducers are not also suited formicrpohone applications, that is, in the reversed sense of translatingfrom an acoustical input to an electrical output. However, it can beappreciated that in a hearing aid application it is necessary to supplyacoutical outputs which are equal to, or greater than, a threshhold offeeling, and this must be accomplished with a diaphragm having a verysmall area. In order to give some idea of the small sizes involved, oneof the most Widely used, present day, miniature transducers fits into acase whose outside dimensions are X X .160".

In the miniaturization of balanced armature transducers, carefulconsideration is required of the basic parameters affecting the design.As is well-known, the output of an electro-acoustic transducer is afunction of the force factor, the effective mass and stiffness of thesystem and effective area of the diaphragm. In general, stiffness is thecontrolling factor affecting output over the low frequency range up tothe resonant frequency of the system.

An inherent advantage of the balanced armature electro-acoustictransducer is the fact that it is possible to maintain mechanicalstability with lower system stiffness than is possible with othervariable reluctance transducers. This decreased system stiffness permitsthe use of a diaphragm With low effective mass and low edge stiffnesswhich, when incorporated with a stiff central portion increases theeffective area of the diaphragm to a high percentage of its total area.All of these characteristics contribute to the high output, as notedpreviously, of the balanced armature transducer.

In order that the diaphragm in a balanced armature transducer willoperate efficiently, With maximum effective area, it is necessary for itto move like a piston. Also, by virtue of its configuration, a balancedarmature transducer requires a mechanical linkage, usually in the formof a drive rod, between the armature and the diaphragm. If a stiff driverod is solidly attached at its ends to these elements the diaphragm willoscillate about its edges. To remedy this condition of oscillation, manymanufacturers of balanced armature transducers use a stirrup drive rod.The stirrup drive rod is made from flat stock and one end is firmlyattached to the diaphragm and the other end, which is made in the shapeof a stirrup, engages the armature with a friction fit, thereby givingsomewhat of a hinged effect. Such a stirrup rod arrangement can beappreciated by reference to the aforenoted Carlson patent 3,111,563.

The stirrup type of drive rod reduces the stiffness at the point ofattachment to the armature and allows the diaphragm to move as a piston.However, this is an arrangement which introduces difficulties withrespect to performance stability. These difficulties come about becauseof the somewhat critical adjustment of the armature in the air gaps,which is necessary to maintain maximum output. If the transducer isdropped in a direction which will result in a force which is largeenough to overcome the friction fit, the drive rod can move along thearmature and thus cause the armature to move out of its adjustedposition. Also, from a production point of view this is a conditionwhich is hard to control. Tools may be made which will insure thefriction fit so as to minimize the unwanted movements of the drive rodafter assembly. This is a condition, however, which will change withtime as the tools become worn.

It has been recognized by me, and this constitutes the genesis of thepresent inventive concept, that the drive rod is the Achilles heel forthe balanced armature transducer. Implementation of my inventive conceptis directed to overcoming the severe problems attendant the driving ofthe diaphragm in a balanced armature transducer. My solution to theseproblems, in essence, provides a means of obtaining a high outputwithout sacrificing the inherent advantages in a balanced armaturetransducer.

Accordingly, it is a primary object of the present invention to providea superior drive rod arrangement in a balanced armature transducer so asto retain great mechanical stability for the system.

Mechanical stability could be improved by firmly attaching the drive rodto the armature rather than using the stirrup connection. This, however,as previously mentioned, will cause in the case of a thick or rigiddrive rod, an unwarranted lateral motion of the diaphragm. With a thindrive rod which is quite short, the stiffness at the point of firmattachment to the armature is reduced and, of course, the armature willdrive the diaphragm with the desired motion. However, for any reasonablelength i.e. when the length of the drive rod is increased so as to begreater than approximately ten times its diameter, it will break up intoa transverse mode of operation at certain frequencies which will causedisturbances in re sponse at these frequencies.

It is, therefore, another basic object of the present invention toprovide a design which permits reasonable drive rod lengths whilepreventing disturbances due to breakup of the drive rod.

In fulfillment of the objects previously recited, it is a feature of theinvention, broadly stated, that the drive rod is so designed as to bemore substantial, to be greater in dimension, in its central portion andto be tapered down at the ends. In one form, the rod is quite thick andtherefore rigid, at its center but is thin, and therefore flexible, atits ends, thereby to provide a hinge action.

More specifically, the low stiffness of the thin drive rod at itsattachment points and the beneficial effect of the thick drive rod whichreduces or eliminates unwanted resonances, are combined by flatteningthe ends of the drive rod and orienting its assembly in the direction oflow stiffness. This calls for having the smaller dimension of theflattened end in line with the longitudinal axis of the armature. Ineffect, then, a more specific feature of the present invention residesin this precise configuration for the drive rod so as to gain, in asimple and efficacious manner, the previously recited advantages.However, this does not constitute the only approach to the desired end.

An ancillary feature, closely related to the design of the drive rod, isthe design of the diaphragm so as to provide very low edge stiffness butto have an overall shape that will avoid certain undesirable effects.Such effects, for example, result from hand slap pressures to which handset receivers are normally subjected. A spherical shape is provided forthe diaphragm so as to result in an improvement in the stability towithstand these hand slap pressures.

When, however, the edge stiffness of the diaphragm is made quite low bythe aforesaid provision for the diaphragm another problem is introducedinto the design. This problem is caused by introducing the possibilityof the diaphragm having two modes of vibration. One will be the desiredpiston-like, vertical motion, and the other will be a sidewaystransverse motion. At certain frequencies, the combination of thesemotions will cause disturbances in response. It is necessary, therefore,to

design the drive rod having the flattened ends in a way which willminimize any disturbances of this type which might occur Within theresponse-frequency pass band.

As has been pointed out hereinabove, the drive rod of the stirrupvariety, which is now used in many balanced armature transducers,eliminates certain unwanted modes of diaphragm vibration. However, theprice paid for the elimination in this manner results in a reduction inmechanical ruggedness which will appreciably reduce the effective lifeof the transducers.

Accordingly, it is a further object of the present invention to providea drive rod design which substantially eliminates undesired diaphragmvibration but which enables an increase in ruggedness for thetransducers.

The previously recited objectives are attained in a specific form by anarrangement of a tubular drive rod having its ends flattened.

It should be noted at this point that the design for the drive rod inaccordance with the present invention will be referred to in thespecific context of a receiver or earphone application, in which contextthe drive rod design has special advantages. However, it will beunderstood that, in general, the drive rod can be applied to a widevariety of transducers in which the recited advantages are desired.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention as illustrated inthe accompanying drawings.

FIG. 1 is a side view in section of a balanced armature transducerillustrative of one embodiment of the present invention.

FIG. 2 is a sectional view taken on the line 2-2 in FIG. 1.

FIG. 3 is a fragmentary view from one end of the driverod-armature-support assembly.

FIGS. 4a, 4b and 4c depict the several conditions that obtain whendifferent drive rods are connected to a diaphragm. FIG. 40 in particulardepicts a drive rod of the present invention.

FIG. 5 shows a number of response curves for balanced armaturetransducers using drive rods, according to the present invention, butvaried in thickness.

Referring now to the drawings, and more particularly, to FIGS. 1-3thereof, an electro-acoustic transducer is illustrated in itsapplication as a receiver.

The transducer 10 comprises a basic frame member 12 to which theelectromagnetic system 14 is affixed and supported. The electromagneticsystem 14 is secured to the inner ring portion 12a, of the frame member12 and the outer ring portion 12b of the member is supported by acup-shaped plastic case 16. The diaphragm 18 is clamped at its peripheryby means of a sealing ring 20 which abuts the periphery of the diaphragmto form a moisture-proof joint with the diaphragm. A cap or cover 22fits over the top of the assembly and engages the peripheral edges ofthe diaphragm 18 and the ring 20', and also engages with a flange on thecase 16. The cap 22 is in the form of a ferrule grid and is providedwith suitable openings, one of which 22a is shown in FIG. 1.

The diaphragm 18 is a laminated diaphragm which consists of two layers18a and 18b. The layer 18a is of aluminum and forms the diaphragmproper. The layer 18b is of Mylar and as seen in FIG. 1 this layer has agreater diameter so that at the periphery of the laminated diaphragm thesupport thereof is provided by clamping the layer 18b.

In order to prevent damage to the diaphragm by any sudden burst ofenergy, a blast stop 24 is provided adjacent the periphery of thediaphragm and is formed as part of the frame member 12. This, of course,limits the excursion of the diaphragm 18 within reasonable limits.

As is typical with the so-called balanced armature type of transducer,movement between the diaphragm and the armature is transmitted by way ofa linkage in the form of a connecting pin or drive rod 26 which isafl'ixed at one end to the center of the diaphragm 18, being attachedthereto by means of an epoxy cement or similar material. At its otherend the drive rod 26 is soldered to the armature 28.

In the application illustrated in FIG. 1 of a receiver or earphone, thediaphragm 18 is driven by the electromagnetic system 14 which iscontained within the cup 16. The electromagnetic system 14 includes amagnetic member in the form of a pair of substantially identical polepieces 32 and 34 and a permanent magnet 36. Each of the pole pieces 32and 34 includes a pair of pole tips or flanges 32a and 32b, and 34a,3412 respectively. The pole pieces 32 and 34 are, of course, in contactwith the permanent magnet means 36 so that a complete magnetic circuitis established. The pole pieces 32 and 34 are spaced in such a way thatthe pole tips 32a and 34a, and also 32b and 34b, are aligned andconfront each other so as to form identical air gaps. The pole pieces 32and 34 are securely clamped by suitable means against the permanentmagnet 36. One means of so clamping would be by means of bolts fitted inthe permanent magnet.

Extending between the pole tips is the magnetic armature 28 having itsends located in the air gaps between respective pairs of confrontingpole tips. Thus, one end of the armature 28 is disposed between the poletips 32a and 34a, while the other end is disposed between the pole tips32b and 34b. In effect, then, four separate air gaps are defined. Thearmature 28 is pivotally mounted at its center in a spring support 40.Preferably the armature 28 passes through a slot 42 formed in thesupport 40 and is secured therein by means of solder. The support 40 isof a material which has good plastic properties and typically, isconstituted of bronze material. The ends of the support 40 are fittedwithin slots 43 in brackets 44. The ends of the support 40 are clampedin the slots 43 by being embedded in an epoxy cement 45 contained withinthese slots. This technique of epoxy clamping of the ends forms-no partof the present invention. However, this feature is described in detailand is claimed in co-pending application Ser. No. 634,572 filed April 28, 1967, assigned to the assignee of the present application.

The brackets 44 are aflixed to the pole piece 32 by any suitable means.Preferably, these brackets are formed in integral fashion as part of thepole piece 32.

The drive rod 26 which extends down into the magnetic member is shownhaving its lower end secured to the armature 28 at the very end of thearmature. However, it

will be appreciated that the location of the drive rod can be variedsomewhat and that it is not essential to the concept of the presentinvention that the position illustrated in FIG. 1 be selected.Alternatively, for example, the end of the drive rod could be fastenedsomewhere along the length of the armature 28 and the same essentialoperation would be obtainable.

The armature 28 is entirely surrounded at one point by a coil 46 carriedby an insulating form or bobbin. This coil may be held in place by theuse of cement or the like. Here, again, although a single coil isdepicted in the drawing, it will be understood that a plurality of coilscan be provided to surround the armature. Conductors (not shown)establish electrical connection to the coil 46 and these conductors aretaken out through a suitable opening such as the opening 50 provided atthe bottom of the casing.

The design of the drive rod 26 is the fundamental feature of the presentinvention. This drive rod, in the embodiment depicted, is so designedthat its center is relatively thick and therefore rigid while its endsare flattened to provide the requisite advantages. Thus, referring tothe drawing, it will be noted that the drive rod 26 has a centralportion of considerable diameter which may vary, for example, between adiameter of .020 and .010 inch.

6 The ends are flattened to have a thickness in the range of .010 to.004 inch.

Referring now to FIGS. 4a, 4b and 4c, the deficiencies that are presentwith drive rods of previous designs are depicted in the upper twoFIGURES 4a and 4b, and the superior results obtainable with the driverod configuration accor'dingto the present invention are shown in thelowermost FIGURE 40. The same reference numerals are used in connectionwith these figures as were used heretofore in explaining the layout ofthe receiver in FIGS. 1-3.

In --FIG. 4a a uniformly thick drive rod 26 is shown. The stiffness atthe points of attachment of the drive rod to th e'armature '28 and thediaphragm 18 is very high. As a consequence, the diaphragm willoscillate as shown around the portion of its edge marked A. On the otherhalf of the cycle the diaphragm will oscillate around the portion ofits-edge marked B.

In contrast with the situation depicted in FLIG. 4a, an entirelydifferent condition obtains in FIG. 4b in which a very thin drive rod isshown connected to the armature and diaphragm. The stiffness at thepoints of attachment to the armature and the diaphragm is low. As aconsequence, were a short drive rod to be used, the diaphragm would moveas a piston. However, when a reasonably long drive rod is actually usedas shown in FIG. 4b, the long drive rod breaks up into other modes ofvibration as depicted and causes the diaphragm to swing from side toside.

The extremely beneficial results that are obtained by the drive roddesign of the present invention are shown in FIG. 40. Here, a thickdrive rod is shown having its ends flattened. The drive rod is assembledto the attachment point with the flattened ends oriented as shown. Thestiffness at these points will be low. The high stiffness of the driverod between these attachment points will reduce the diaphragm break upin the frequency-response pass band and the diaphragm will move as apiston.

A number of drive rods having the same general configuration as depictedin FIG. 40 were tested. Data showing the effects of these drive rodswith flattened ends are shown in RIG. 5. These data were taken with thereceiver already described and shown in FIG. 1. The curves that areplotted in FIG. 5 show the relationship between the response in db andthe frequency in cycles per second. Three sets of curves are shown inFIG. 5. The curve in each set which is shown dotted represents theearphone response without benefit of acoustic circuit, whereas the solidcurve represents the earphone response with benefit of acoustic circuit.The upper set of curves were taken for a drive rod having an endthickness of approximately .009 inch. The middle and lower set of curvesare for end thicknesses of .007 inch and .006 inch respectively. Theparticular drive rod for which the curves were taken was one made fromseamless brass tubing having an outside diameter of 0.015 inch and aninside diameter of .009 inch. Thus, the wall thickness of the tubularrod is 0.003 inch.

The response of the earphone without acoustic circuits represented bythe dotted lines shows the effects of the transverse motion of thediaphragm in the region of 2500 to 3000 c.p.s. for all of these driverods. However, these data show that the magnitude of the disturbancedecreases as the end thickness is decreased and that when the responsewith acoustic circuits is measured the disturbance is completely dampedout in the case of the thinnest sample, i.e., .006 inch end thickness.Thus, in this particular sample the tubular rod ends were flatteneduntil the opposed side walls were abutting against each other, that is,until the thickness of the rod was equal to twice the thickness of thetube side wall.

While there have been shown and described and pointed out thefundamental novel features of the invention as applied to the preferredembodiments, it will be understood that various omissions andsubstitutions and changes in the form and details of the deviceillustrated and in its operation may be made by those skilled in the artwithout departing from the spirit of the invention. It is the intention,therefore, to be limited only as indicated by the scope of the followingclaims.

What is claimed is: 1. A balanced armature transducer, comprising: (a) acircular diaphragm connected at its periphery to a fixed support havinga central portion movable as a piston with respect to said support; (b)electromagnetic means for actuating the diaphragm (1) a magnetic circuitincluding a pair of air gaps; (2) electrical winding means linked tosaid magnetic circuits; (3) an armature supported in said air gaps andmovable therein; and (c) means connecting one end of the armature to thecenter of the diaphragm for concurrent movement therewith; wherein theimprovement comprises: (d)said connecting means consisting of anintegral drive rod formed of tubular metal having both end portions onlyflattened with their short dimensions parallel to the longitudinal axisof said armaturew 2. A balanced armature transducer as defined in claim1, wherein said drive rod has a tubular central portion, and the endportions at which the walls of the tube are 5 flattened against eachother.

References Cited UNITED STATES PATENTS G. HARRIS, Primary Examiner.

US. Cl. X.R.

